The present invention relates to the use of facings, having high air flow resistances, to enhance the sound absorption properties of certain porous insulation materials and especially, those insulation materials having low air flow resistances. By laminating the high air flow resistance facings to these particular porous insulation materials, these porous insulation materials exhibit sound absorption characteristics normally provided by more costly fibrous insulation materials of greater thickness, higher bulk density and/or smaller average fiber diameter and foam insulation materials having smaller cells and pores.
Porous insulation materials such as thermoplastic glass or polymeric fiber blankets and polymeric foams are used in many applications to enhance the sound absorption performance of various products and systems. Typical applications include: acoustical wall panels, ceiling panels and office partitions; automotive headliners and hoodliners; liners for heating, ventilating and air conditioning systems; appliance insulation; and similar applications.
The sound absorption characteristics of these porous insulation materials is a function of the acoustic impedance of the material. The acoustic impedance is a complex quantity consisting of frequency dependent components called, respectively, acoustic resistance and acoustic reactance. The acoustic reactance of these porous insulation materials is governed largely by the thickness of the product and, to a much lesser extent, by the mass per unit area of an air permeable facing or film which may be applied over the surface of the porous insulation material. The acoustic resistance of the porous insulation material is governed by the air flow resistance of the porous insulation material.
The ratios of the acoustic reactance and the acoustic resistance to the characteristic impedance of air determines the normal incidence sound absorption coefficient. For a given value of the acoustic reactance ratio, there is an optimum value of the acoustic resistance ratio which will provide the maximum sound absorption. Since the reactance ratio of a porous insulation material is determined largely by the thickness of the porous insulation material, the most effective way of controlling the sound absorption properties of a porous insulation material is by adjusting the acoustic resistance ratio. In the past, the acoustic resistance ratio has been adjusted by changing the physical properties of the porous insulation materials. In fibrous insulations, such as glass fiber insulations, the average fiber diameter of the insulation has been decreased, the bulk density of the insulation has been increased, and the binder content of the insulation has been increased. In polymeric resin foam insulations, the average pore or cell size of the insulation material has been decreased. While these physical modifications increase the acoustic resistance ratio of these insulation product, the cost of producing these products is also increased.